1. Field of the Invention
The present invention relates to electronic ballasts for metal halide lamps, and more particularly to an electronic ballast for metal halide lamps having a three-stage structure. The three-stage electronic ballast for metal halide lamps mainly comprises a step-up converter, a step-down converter and a full-bridge DC-AC converter, wherein the step-down converter operates an inductor in a continuous boundary current mode to achieve reducing power loss and enhancing efficiency. Equipped with a micro processor, the electronic ballast further possesses the function of power regulation. The electronic ballast can be added with various protective functions without complex control circuits and sensing elements, thereby becoming a high-quality and low-cost electronic ballast for metal halide lamps.
2. Description of the Prior Art
High-intensity gas-discharging lamps are widely used as light sources for indoor or outdoor illumination. Given its intensity, efficiency, duration and color, metal halide lamps have been used in department stores, supermarkets or in advertisement boards for providing high quality illumination.
The majority of the electronic ballasts for metal halide lamps of the prior art include a power factor correction circuit made of a step-up converter, a DC—DC step-down converter, a full-bridge DC-AC converter and an ignition circuit. Although circuitry structure of the prior art is similar to the three-state structure according to the present invention, its step-down converter is operated in a discontinuous current mode to lessen running down of switching elements. Because of appreciable variation in the characteristic of a metal halide lamp over running time and different manufacturers, it requires a wider design margin for operating the inductor of a step-down converter in discontinuous current mode. This results in high peak values of the inductor currents and therefore high power loss in the circuit. And, consequently, the electronic ballasts of the prior art have step-down converter of low efficiency and high operating temperature, which makes the electronic ballasts less durable.